<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>
        /* body总是有一个默认的外边距margin: 0,常见操作 */
        h1,ul,li,a,body{
            margin: 0;
            padding: 0;
            text-decoration: none;
        }
        #box {
            width: 300px;
            border: 1px solid green;
        }
        h2{
            font-size: 16px;
            background-color: #3cbda6;
            line-height: 30px;
        }
        form{
            background: cornflowerblue;
        }
        div:nth-of-type(1) > input{
            border: 3px solid black;
        }

        div:nth-of-type(2) > input{
            border: 3px dashed black;
        }

    </style>
</head>
<body>

<div id="box">
    <h2>会员登录</h2>
    <form action="#">
        <div>
            <spam>用户名：</spam>
            <input type="text">
        </div>
        <div>
            <spam>密码：</spam>
            <input type="text">
        </div>
        <div>
            <spam>邮箱：</spam>
            <input type="text">
        </div>
    </form>
</div>

</body>
</html>